The large heat exchangers used by commercial coal-fired power plants are prone to tube leaks. Tube leaks represent a potential for serious physical damage due to escalation of the original leaks. For instance, the steam tubes located in the superheat/reheat section of a boiler are prone to cascading tube failures due to the close proximity of the steam tubes coupled with the high energy of the escaping steam. When undetected for an extended time, the ultimate damage from serious tube failures may range from $2 to $10 million/leak, forcing the system down for major repairs that can last up to a week.
If detected early, tube failures may be repaired before catastrophic damage, such repairs lasting only several days and costing a fraction of the cost associated with late detection and catastrophic damage. Repair times may be further reduced if the location of the leak is identified before repairs are initiated. In addition, accurate location allows the operator to delay shutdown and repair of leaks that occur in less critical regions of the boiler, such as the water wall, until economically advantageous.
Boiler tube leaks result in the diversion of water from its normal flow paths as the coolant in the boiler, directly into the combustion environment. The amount of water typically diverted by a leak is small relative to the normal variations in feed water flow rates and sources of water in the fuel/air mixture. Other sources of water in the fuel/air mixture are myriad and subtle including: water added at the point of combustion as steam used to atomize fuel; water used by pollutant control processes; water used in soot blowing; water formed from the combustion of hydrocarbon fuels; free water born by the fuel; and moisture carried by combustion air. These confound the discrimination of boiler tube leaks by a variety of prior art methods that have been employed in an attempt to detect them. In addition, the normal operation of the plant is subject to seasonal variation, variation in the quality of the combustion fuel, and manual operator choices, making it extremely difficult to detect boiler tube leaks in their incipient stages.
A system and method has been proposed in U.S. patent application publication No. 2005/0096757 for detecting faults in components of a continuous process, such as a boiler. A model of the process is developed using a modeling technique such as an advanced pattern recognition empirical model, which is used to generate predicted values for a predetermined number of the operating parameters of the process. The operating parameters in the model are drawn from the sensors that monitor the flow of steam/water through the balance-of-plant (“BOP”). The BOP encompasses the components of a power plant that extract thermal energy from the steam/water mixture and convert it to electrical energy. As such, the BOP excludes the boiler itself. The model monitors flow rates of steam/water entering into and exiting from the BOP, which correspond to the flow rate of superheated steam from the top of the boiler and the flow rate of condensed feed water into the bottom of the boiler, respectively. Under normal conditions, the flow entering the BOP is balanced by the flow exiting the BOP. One of the abnormal conditions that can upset this balance is a boiler tube leak. This approach, built around a mass and energy balance on the BOP, is capable of indirectly detecting a boiler tube leak. But since the model does not monitor any operating parameter internal to the boiler, including any parameter from the fuel/air side if the boiler, it is incapable of locating a tube leak.
What is needed is a way of monitoring a heat exchange environment in a fossil fuel power plant that is sensitive enough to detect boiler tube leaks in their initial stages from existing instrumentation present in the plant.